1. Field of the Invention
The present invention relates to a high (or highly) light transmissive dust-proof body allowing an excellent transmission of light, and a method of preparing the same. More specifically, it relates to a dust-proof body to be used for preventing a contamination by dust and foreign matter, etc., of a photomask and reticle (hereinafter referred to as a mask) to be used in the photolithographic step of the preparation of semiconductor elements such as IC and LSIs, etc., and a method of preparing same.
2. Description of the Related Art
In the photolithographic step, a mask having a circuit pattern formed of a film of chromium vapor deposited on a glass plate surface is used for transferring the circuit pattern onto a silicon wafer having a resist coated thereon. In this step, when an exposure is effected under conditions in which foreign matter such as dust, etc. can become attached to the circuit pattern on the mask, the above foreign matter is also transferred onto the wafer, and thus the product is defective. Particularly, when the above-mentioned exposure is effected by a stepper, it is extremely probable that all of the chips formed on the wafer are defective, and therefore, the contamination by foreign matter of the circuit pattern of the mask is a serious problem. To solve this problem, recently a dust-proof body (pellicle) having a transparent light-transmissive film arranged at an appropriate distance from one surface or both surfaces of the mask substrate has been used.
Such a dust-proof body generally comprises a transparent light-transmissive film of an organic material comprising nitrocellulose fixed on one side surface of a holding frame made of aluminum: the other end surface thereof being plastered with a double-sided sticky tape so as to be mountable onto a mask substrate. The use of this film prevents a penetration of foreign matter from outside, and even if foreign matter is attached to the film it will not be transferred onto a wafer, thus providing an improvement of the yield during the production of semiconductor elements.
As the transparent film constituting this dust-proof body, a single layer thin film of nitrocellulose is primarily utilized in the prior art, and to improve throughout in the exposure step, there has been proposed a dust-proof film having a reflection preventive layer comprising a laminated film composed of a high refractive index layer and a low refractive index layer provided on the transparent thin film of nitrocellulose (Japanese Unexamined Patent Publications (Kokai) Nos. 60-237450, 61-53601, 61-209449, 62-127801). Here, as the high refractive index layer forming the reflection preventive layer, polystyrene, polysulfone, polyether sulfone, polyphenylene ether, polycarbonate, aromatic polyester, polyvinylnaphthalene, are used.
Nevertheless, improvements in the degree of integration of semiconductor elements have shifted the light used for the exposure toward a shorter wavelength, from a g-beam (436 nm) to an i-beam (365 nm), and further, to an excimer laser (248 nm), and therefore, a problem arises in that the above-mentioned organic transparent film or the above-mentioned high refractive index layer of the organic material is deteriorated, whereby another problem arises in that the dust-proof film can no longer withstand practical application.
Further, the improvements in the degree of integration of semiconductor elements, which have caused a shift of the light during exposure toward a shorter wavelength, i.e., from a g-beam (436 nm) to an i-beam (365 nm), and further to an excimer laser (248 nm), have given rise to another problem in that photolysis occurs in a dust-proof body using the transparent thin film of an organic material of the prior art, and thus it can no longer be used for practical application.